The application relates to a touch detection device which detects an external proximity object such as a finger, to a display device having a touch detection function, to an electronic unit, and to a touch detection circuit used for those devices.
In recent years, attention has been drawn to a display device in which a contact detection device, generally referred to as a touch panel, is mounted on a display such as a liquid crystal display, or in which the touch panel and the display are integrated, and which allows the display to display various button images etc. to thereby make it possible to input information instead of providing typical mechanical buttons. The display device provided with the touch panel eliminates a necessity of providing an input device such as a keyboard, a mouse, and a keypad, and is thus being used more and more not only in a computer but also in a portable handheld terminal such as a cellular telephone.
There are a number of types of touch panels, including such as an optical type and a resistance type. In devices such as the portable handheld terminal, in particular, there are high expectations for a touch panel of an electrostatic capacitance type. The electrostatic capacitance type touch panel has a relatively simple configuration, and is capable of reducing power consumption. However, the electrostatic capacitance type touch panel is, in principle, susceptible to noise attributed to an inverter fluorescent lamp, an AM (amplitude modulation) wave, an AC (alternating current) power source, or other noise source (hereinafter referred to as an “external noise”). In particular, a detection sensitivity is likely to be decreased in detecting a proximal state or a “proximity” of an external proximity object, since a signal-to-noise (S/R) ratio deteriorates as compared with a case in which a contact state of the external proximity object is to be detected.
Various studies have been made with a view toward providing a method for improving a resistance to the external noise. For example, Japanese Unexamined Patent Application Publication No. 2007-13432 (JP2007-13432A) discloses a touch panel in which electrostatic sensors, dedicated for detecting the external noise, are provided around and provided separately from an input electrostatic sensor used for detecting the proximity or the contact of the external proximity object. The disclosed touch panel compares signals obtained from the noise-detecting electrostatic sensors and the input electrostatic sensor to separate a touch component from an external noise component. A surface of the noise-detecting electrostatic sensor is provided with a protection cover, so that the noise-detecting electrostatic sensor makes no response to the external proximity object even when the external proximity object is in contact thereto.